1. Field of the Invention
The present invention relates to a tracking control apparatus and an optical pickup having the same. More particularly, the present invention relates to a tracking control apparatus for compensating tracking error signals by controlling a position of a laser beam irradiated onto tracks of an optical disk and an optical pickup having the same.
2. Description of the Prior Arts
Data recording/reproducing apparatuses using optical disks such as a laser disk (LD) and a compact disk (CD) have been commercially available in recent years. In order to read out data from an optical disk, a laser beam is irradiated onto a data recording track (to be referred to as a track hereinafter), and data are reproduced based on the beam reflected by the track. When the tracks are helically formed on the optical disk, since the sectors of a single track are not equidistant from the center of the rotation of the disk, tracking (radial) control is necessary in the read mode to accurately irradiate the track with a laser beam. Even if the tracks are concentrically formed, sectors of a single track are not equidistant from the center of the rotation of the disk due to the eccentricity in the disk, and therefore, tracking control is essential. This tracking control is conventionally performed by using a push-pull method (one beam method) or three beam method (out-trigger method).
As a typical conventional example of a tracking circuit, a lens tracking circuit may be mentioned wherein an objective lens is moved in response to a tracking error signal derived from the beam reflected by or transmitted through an optical disk, e.g., one beam or three beams formed from a beam irradiated from the laser by the one beam method or three beam method. The objective lens is normally supported and fixed by a spring on an optical head housing. A tracking actuator is energized to move the lens for the tracking control. When the tracking actuator is deenergized, the lens is held at a mechanically neutral point balanced by a spring force.
When the track eccentricity exceeds twenty or thirty microns, the objective lens deviates greatly from the mechanically neutral point, thereby combining an optical offset signal with the tracking error signal. The laser beam then traces a wrong track in response to the optical offset signal.
In order to eliminate the optical offset signal, a tracking system called a two-step servo system has been developed, as described in U.S. Pat. No. 4,761,773. According to this system, the carriage and hence the optical pickup itself as well as the objective lens are moved to perform cooperative tracking.
Meanwhile, since the distance from the optical pickup to disk shifts minutely in the read mode in which the disk is rotated, it is difficult to correctly read data due to the shift, thus rendering focusing control essential. This focusing control is conventionally performed by astigmatic method using astigmatism or a knife edge method.
As a typical conventional example of a focusing circuit, a lens focusing circuit may be mentioned wherein an objective lens is moved in response to a focusing error signal derived from laser beam which is irradiated from laser source and then reflected by or transmitted through an optical disk. The objective lens is normally the same lens that is used for the tracking control. A focusing actuator is energized to move the lens for the focusing control. When the focusing actuator is deenergized, the lens is held at a mechanically neutral point balanced by a spring force.
The conventional optical pickup having the tracking control apparatus will be described in detail with reference to FIGS. 1A and 1B.
FIG. 1A is a schematic view for showing a conventional optical pickup. As shown in the figure, a light source 10 has a laser diode to generate a laser beam. In the case of the three beam method, grating 18 is provided at the front or the rear of a collimator lens 11, that is, between laser source 10 and collimator lens 11 or collimator lens 11 and a beam splitter 12, which separates one beam into three beams. The laser beam irradiated from laser source 10 is changed into parallel beams by collimator lens 11. These parallel beams, in the three beam case, are separated into three beams by grating 18 and then pass through the beam splitter 12, a .lambda./4 plate 13, and an objective lens 14 to be incident upon the surface R of the disc D to form a beam spot of about 1 .mu.m.
Beam splitter 12 has two right-angled prisms of which oblique (45.degree.) facets meet with each other. On the oblique facet, a polarizing film is formed so that while ensuring the straight property of the incident beam, a part of the incident beam transmits the prisms and the other part is reflected from the polarizing film at an angle of 90.degree. with respect to the incident beam. Additionally, .lambda./4 plate 13 serves to avoid interference of the incident beam and the reflective beam, and transforms a linear polarization into a circular polarization or inversely using double refraction phenomena.
The intensity of the light reflected from the disk D depends on existence of a pit thereof having recorded data. The recorded information is read out on the basis of the intensity of the reflected light. The reflected light is transformed into the parallel light while going through the objective lens 11, polarized by 90.degree. at .lambda./4 plate 13, and then is incident onto beam splitter 12. In beam splitter 12, one part of the incident beam is reflected at 90.degree.. A converging lens 15 is placed at the optical path of the reflected light to converge the reflected light. The reflected light converged by converging lens 15 goes through cylindrical lens 16 (or knife edge) and then is received by a four or six partitioned light-receiving diode 17. The position errors of the pickup apparatus with repect to the disk D including a focusing error and a tracking error are detected from the image of the beam received on light-receiving diode 17, and focusing error signals and tracking error signals are generated according to these errors. A voice coil motor 19 as the objective lens actuator is actuated to move the objective lens in response to the error signals, so that focusing and tracking can be controlled. The information on the disk is reproduced on the basis of the intensity of the reflected light which is determined by pit (P) on the disk (D).
According to the conventional optical pickup, when the focusing error or tracking error is detected by light-receiving diode 17, voice coil motor 19 is actuated to move objective lens 14 in a horizontal or vertical direction, so that the laser beam can be focused and tracked accurately on the surface of the disk D. That is, in the case of focusing control, a focusing coil part of voice coil motor 19 is energized to move objective lens 14 in the vertical (axial) direction. On the other hand, in the case of tracking control, the tracking coil part of voice coil motor 19 is energized to move objective lens 14 in the horizontal (radial) direction.
When voice coil motor 19 is operated by the two-axially actuating method in this way, there is a problem that a damping process has to be performed in order to avoid the occurrence of a resonance within the resonance frequency bandwidth since voice coil motor 19 has its own resonance frequency.
Furthermore, as shown in FIG. 1B, the tracking and focusing servo bandwidths are narrow, i.e., the higher the frequency is increased (applied to voice coil motor 19 which is the actuator of the objective lens), the more the sensitivity (the amplitude of the beam spot on the disk to the frequency) of voice coil motor 19 deteriorates. Therefore, there is another problem when responding to a high-frequency tracking error signal occurring at far higher frequency than the resonance frequency, e.g., occurring at a multiple kHz frequency, that tracking errors cannot be compensated accurately since the sensitivity is fairly low in such frequency band.
Furthermore, there is another problem that the tracking servo cannot be carried out smoothly when a large impact is applied during the reproduction of the disk D.